As energy companies began to sink horizontal wells into the Barnett Shale of north-central Texas a decade ago, a distinctive production curve appeared that has become a strong visual symbol in the debate about the future of the vast U.S. shale gas resources.

The only shale gas play with a decade's track record is the Barnett Shale, found more than a mile below the Dallas-Fort Worth region, where the pioneering work of George Mitchell and his Mitchell Energy Co. established the potential of horizontal drilling and hydraulic fracturing.

Shale gas production took off in the Barnett formation around 2005, and by the next year, production totaled 3.1 billion cubic feet a day, a significant fraction of the nation's consumption, averaging 23 bcf per day. The total jumped to 5.6 bcf in 2011 before flattening out last year, according to Bentek Energy reports. January production from Barnett dropped to about 5.3 bcf, said Bentek analyst Ryan Smith.

The economic impact has been dramatic. Gas production from the Barnett Shale added $133 billion to the Texas economy in 2008 alone and has helped create more than 99,000 jobs, according to the industry-supported energyfromshale.com.